1. Field of the Invention
The present invention relates to an oil supply unit for a running power transmission of a vehicle. The oil supply unit includes a hydraulic pump which is operatively connected to an engine and which supplies discharged oil to a power transmission portion in a power transmission mechanism.
2. Related Art
An oil supply unit including a hydraulic pump operatively connected to an engine is conventionally utilized in various fields such as a working vehicle.
For example, the oil supply unit is formed so as to supply discharged oil from the hydraulic pump as hydraulic fluid and lubricating oil for a hydraulic actuator such as a forward/reverse switching device and/or a PTO clutch and, also, to supply the discharged oil as lubricating oil for a member to be lubricated such as a mechanical transmission which requires lubrication.
However, there is a problem of large power loss in the conventional oil supply unit, because a discharge quantity of the hydraulic pump is merely proportional to an engine speed.
In other words, in the conventional oil supply unit, the hydraulic pump is of a fixed displacement type; therefore, a discharged oil quantity of the hydraulic pump is simply proportional to the engine speed. In short, the discharge quantity of the hydraulic pump is the smallest when the engine is at idle and increases as the rotation speed increases from idle.
The oil quantity discharged from the hydraulic pump is set according to use of the discharged oil.
For example, an assumption is made that the discharged oil is utilized as hydraulic fluid for the hydraulic actuator such as the forward/reverse switching device and the PTO clutch and is also utilized as lubricating oil for the members to be lubricated such as the hydraulic actuator and the mechanical transmission.
In this case, if an amount of hydraulic fluid required by the hydraulic actuator is X1 m3/s and an amount of lubricating oil required by the members to be lubricated is X2 m3/s, the hydraulic pump is set to be able to discharge oil of X1 m3/s+X2 m3/s when the engine is at idle.
Because the discharge quantity of the hydraulic pump is proportional to the engine speed in the conventional oil supply unit as described above, the hydraulic pump discharges oil more than the required discharged amount X1 m3/s+X2 m3/s when the engine speed is increased. The surplus discharged oil merely circulates through an oil hydraulic circuit and is not used advantageously.
Because the hydraulic pump of the fixed displacement type is set to discharge the required amount of oil when the engine is at idle in the conventional oil supply unit as described above, if the engine speed is increased, useless oil according to the increase is discharged, which causes the power loss.
It has conventionally been proposed to utilize a housing for accommodating the power transmission mechanism as an oil source of the hydraulic pump.
With this structure, the housing is also used as an oil reservoir to thereby save space and reduce costs by using the member in two ways.
However, the oil stored in the housing causes stirring resistance to the power transmission housed in the housing, which results in further power loss in the above structure.
The present invention has been accomplished with the above conventional art in view and it is an object of the invention to reduce power loss in an oil supply unit formed to use discharged oil from a hydraulic pump operatively connected to an engine as hydraulic fluid and/or lubricating oil.
It is another object of the invention to prevent running out of oil and to reduce power loss when a housing for accommodating a power transmission mechanism is used as an oil source of the hydraulic pump.